Information is more rapidly processed and transmitted using optical as opposed to electrical signals. There exists a need for finding nonlinear optical materials, and processes for preparing such materials, which alter the transmission of optical signals or serve to couple optical devices to electrical devices, i.e., electrooptic devices.
Some materials which have been used in electrooptic devices include semiconductors such as lithium niobate, potassium titanyl phosphate and gallium arsenide and most recently, organic materials which have been doped with nonlinear optical materials. Generally, polymeric organic materials can or may have the specific advantages of fast response time, small dielectric constant, good linear optical properties, large nonlinear optical susceptibilities, high damage threshold, engineering capabilities, and ease of fabrication.
There are various known polymeric organic materials which possess specific nonlinear optical properties and various known processes for making such polymeric organic materials. Many of the current polymeric organic materials prepared by the prior art are prepared by blending a NLO molecule into a polymer host material. "Blending" herein means a combination or mixture of materials without significant reaction between specific components.
Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D.C. 183. The above-recited publications are incorporated herein by reference.
EP 218,938 discloses one method of making a polymer with nonlinear optical properties by incorporating molecules which have nonlinear optical (NLO) properties into a host polymer. The NLO molecules are incorporated into the host polymer by blending. The NLO molecules in the polymer can be aligned by an electric field while the temperature of the polymeric material is raised above its glass transition temperature and then cooled to room temperature. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of molecules which have NLO activity including azo dyes such as disperse red 1.
PCT Application WO8802131A also describes a method of blending a substance having nonlinear optical properties, such as 2-methyl-4-nitroaniline, into a commercially available curable epoxy resin polymer to prepare an electrooptical material.
It is also known to incorporate a NLO active group such as azo dye Disperse Red 1 (4,-[N-ethyl-N-(2-hydroxyethyl] amino-4-nitro azobenzene), by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described in Applied Physics Letters 49(5), 4 (1986). In this paper, an aromatic amine is disclosed but the amine is not covalently bonded to the polymer chain. In addition, the paper discloses an NLO molecule which has an electron donor and acceptor group at either end of the molecule.
A problem associated with a polymer with NLO properties produced by simply blending NLO molecules into a host polymer is that these polymer materials lack stability of orientation, i.e., there is a great amount of molecular relaxation or reorientation within a short period of time resulting in a loss of NLO properties. For example, as reported by Hampsch et al., Macromolecules 1988, 21, 528-530, the NLO activity of a polymer with NLO molecules blended therein decreases dramatically over a period of days.
Generally, the incorporation of molecular structures which have NLO activity into the backbone of a polymer chain will decrease the likelihood of the structural reorganization in comparison with polymers in which the NLO active molecule is simply blended. It is therefore desirable to provide a polymer material with NLO groups covalently bonded to the backbone of the polymer material to minimize relaxation effects.
U.S. Pat. No. 4,703,096 discloses a polymer composition in which the NLO activity is derived from aromatic structures attached to a polymeric diacetylenic backbone. However, the synthesis of the material described in U.S. Pat. No. 4,703,096 is complicated.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects. It would be highly desirable to have organic polymeric materials, particularly polymeric materials based on epoxy resins, with larger second and third order nonlinear properties than presently used inorganic electrooptic materials.
It is desired to provide a NLO molecule with ends having both donors with an acceptor being in the middle. It is further desired to tie both ends of a NLO active molecule into a polymer chain to provide enhanced stability over other NLO molecules in which only one end is tied to the polymer backbone.
There are two main problems which are associated with polymeric nonlinear optical materials. The first problem is dilution of the NLO effect of a polymer. Dilution of the NLO effect occurs, for example, when a molecule possessing nonlinear optical coefficients is added to a host material having only linear optical properties. As an illustration, it has been shown, in U.S. patent application Ser. No. 441,783, filed of even data herewith by J. J. Kester, that materials like diamino diphenylsulphone (DADS) and oxydianiline (ODA) exhibit a nonlinear optic susceptibility. When the nonlinear optical molecules, such as DADS or ODA, are incorporated into a crosslinked structure of an epoxy polymer, films with second and third harmonic generating capabilities are produced. However, the concentration of NLO molecules in these crosslinked polymers may be diluted by the presence of an epoxy resin which has very low NLO susceptibilities relative to the DADS and ODA.
The second problem associated with polymeric nonlinear optical materials is relaxation of orientation of NLO groups within an oriented polymer due to thermal processes.
The relaxation of the NLO effect has been documented in the literature, for example, in the aforementioned reference H. Hampsch et al., Macromolecules 21, p. 528-530. These relaxation can occur at room temperature and be well below the glass transition, Tg, of the polymer.
Monomers having glycidyl groups such as tetraglycidylsulfonyldianiline, are disclosed in the reference, W. T. Hodges et al., "Evaluation of Experimental Epoxy Monomers", SAMPE Quarterly, Volume 17, No. 1, Oct. 1, 1985, pp 21-25.
It would be highly desirable to provide an epoxy monomer having nonlinear properties and to provide epoxy based polymeric nonlinear optical materials with improved NLO properties and improved relaxation properties.
An object of the present invention is to provide an epoxy based polymers which exhibit nonlinear optical effects and which have enhanced stability of nonlinear optical effects.